Samurai start with a rustproof splint mail, and splint mail generated on the Samurai quest will also be rustproof. As an odd side effect, when the player is a Samurai, any random splint mails generated on level 1 at the start of the game will also be rustproof.

Origin

Splint mail is a standard type of body armor in Dungeons & Dragons and games and media based on it. It is made of metal "splints".
Historically "splint armor" can mean either armor made of long parallel strips (a common design for armor for the arms and legs, but not the torso), or "plated mail", made of small rectangular plates attached to a chain mail backing. Compare banded mail, which is sometimes described as made of strips of metal arranged horizontally on the body.

Plated mail was common in Asia and eastern Europe. The splint mail worn by NetHack's samurai is probably tatami, made of plates embedded in chain mail. A comment in the source code[1] indicates that the rustproofing represents the historical use of lacquer to protect Japanese armor in a wet climate.